The invention relates generally to continuous gas fluidized bed methods for making a polymer featuring a condensing agent in a recycle stream; and also to methods for monitoring and providing continuity in a gas fluidized bed method for making a polymer featuring a condensing agent in a recycle stream.
In general, polymerization reactions are exothermic. Therefore, producing a polymer in a fluidized bed necessitates that the heat generated by the polymerization reaction be removed in order to keep the reaction temperature within the bed in a desirable range. Conventionally, the temperature of the reactor fluidized bed is controlled to an essentially isothermal level through continuously removing the heat of polymerization by circulating the gas exiting from the fluidized bed to a condenser/heat exchanger outside the reactor and recirculating the cooled gas stream back into the reactor. When the temperature of the recirculating stream introduced or recycled into the fluidized bed polymerization reactor is above the dew point temperature, substantially no liquid is present and, thus, such operation is known in the art as a xe2x80x9cdry modexe2x80x9d process. However, it has been recognized that the recycled stream need not be completely gaseous but can comprise both gas and liquid. In this process, fluid is formed by cooling the recycle stream below the dew point temperature, thereby converting a portion of the gas into a liquid, and the cooled recycle stream is introduced into the fluidized bed polymerization reactor. This mode of operation is known in the art as a xe2x80x9ccondensing modexe2x80x9d or xe2x80x9ccondensed modexe2x80x9d process.
Condensed mode fluidized bed reactor polymerization processes have been disclosed by, e.g., U.S. Pat. Nos. 4,543,399 and 4,588,790 to Jenkins et al., each of which describes introducing an inert liquid into the recycle stream to increase the dew point temperature of the recycle stream and allowing the process to operate at levels of up to 17.4% liquid by weight, based on the total weight of the cooled recycle stream. A condensed mode process is considered to be advantageous because its ability to remove greater quantities of heat generated by polymerization increases the polymer production capacity of a fluidized bed polymerization reactor.
Condensed mode fluidized bed reactor polymerization processes operating with above 17.4% liquid by weight in the cooled recycle stream have been disclosed; however, such processes must be confined within a limited range of operating conditions to avoid destabilizing the fluidized bed, thereby halting the process. For example, the teachings of U.S. Pat. No. 5,352,749 to DeChellis et al. require that the ratio of the fluidized bed density (xe2x80x9cFBDxe2x80x9d) to the settled bulk density (xe2x80x9cSBDxe2x80x9d), the latter being the density of the polymer particles produced, be maintained above 0.59 throughout the fluidized bed polymerization process. In particular, this reference discloses that xe2x80x9cas a general rule a reduction in the ratio of FBD to SBD to less than 0.59 may involve risk of fluidized bed disruption and is to be avoided.xe2x80x9d
Different, but still limited, ranges for condensed mode fluidized bed polymerization reactor operating conditions are disclosed in U.S. Pat. Nos. 5,436,304 and 5,462,999, each to Griffin et al. Each of these references defines a so-called xe2x80x9cbulk density function (Z)xe2x80x9d (see, e.g., col. 12, lines 31-42 of U.S. Pat. No. 5,436,304 for a definition of Z) and teaches that Z is to be maintained at a value equal to or greater than a so-called xe2x80x9ccalculated limit of the bulk density functionxe2x80x9d (see, e.g., col. 12, lines 66-68 in U.S. Pat. No. 5,436,304) throughout the fluidized bed polymerization process to avoid destabilizing the fluidized bed.
Yet another different, but still limited, range for condensed mode fluidized bed polymerization reactor operating conditions is disclosed in U.S. Pat. No. 6,391,985 B1 to Goode et al. This reference teaches operating the fluidized bed polymerization process with at least 17.5% liquid by weight in the cooled recycle stream and in the so-called xe2x80x9cturbulent regime,xe2x80x9d defined (see, e.g., col. 4, lines 20-27 and 52-54; col. 2, lines 3-9) as xe2x80x9cthe state of a fluidized bed existing between the conditions of (1) the presence of discernable bubbles and (2) fast fluidization, and/or the regime of conditions between (a) the transition velocity Uc and (b) the transport velocity Uk, expressed as the superficial gas velocity.xe2x80x9d
There remains, however, a need for identifying a broader range of operating conditions of a condensed mode fluidized bed reactor polymerization method. Confining a condensed mode fluidized bed reactor polymerization process within limited operating ranges, e.g., those discussed above, to avoid destabilizing the fluidized bed prevents the benefits of condensed mode operation from being fully realized. Moreover, none of the above references even suggests focusing on, e.g., the difference between the lower fluidized bed density and the upper fluidized bed density, in a method for making a polymer featuring a condensing agent in a recycle stream and/or in a method for monitoring and providing continuity in a gas fluidized bed method for making a polymer featuring a condensing agent in a recycle stream.
The citation to any reference in Section 2 of this application is not an admission that any such reference is prior art to this application.
An embodiment of the invention relates to a continuous gas fluidized bed method for making a polymer featuring a condensing agent in a recycle stream comprising:
continuously passing a gaseous stream comprising monomer and the condensing agent through a fluidized bed in a reaction zone having a controlled reactor bed temperature, a lower fluidized bed density, an upper fluidized bed density and a plurality of measuring siten temperatures, in the presence of catalyst;
withdrawing from the reaction zone polymer product and a stream comprising unreacted gases;
recycling the stream into the reaction zone with sufficient additional monomer to replace monomer polymerized and withdrawn as polymer product;
cooling the recycle stream to condense a portion thereof and form a liquid-containing mixture having a recycle stream dew point temperature, a reactor inlet temperature and comprising from about 17.5% to about 70% liquid by weight based on the total weight of the cooled recycle stream where the difference between the controlled reactor bed temperature and the recycle stream dew point temperature of the mixture is greater than or equal to about 5xc2x0 C.; and
introducing the mixture into the reaction zone where the liquid in the mixture is vaporized;
where xcex94xcfx81 satisfies the condition 0 kg/m3xe2x89xa6xcex94xcfx81 less than 70 kg/m3 and, when xcex94xcfx81xe2x89xa710 kg/m3, simultaneously, at least a critical number of An satisfy the condition 0.25xe2x89xa6Anxe2x89xa60.8.
Another embodiment of the invention relates to a method for monitoring and providing continuity in a gas fluidized bed method for making a polymer featuring a condensing agent in a recycle stream comprising:
monitoring the fluidized bed reaction zone, where the reaction zone has a controlled reactor bed temperature, a lower fluidized bed density, an upper fluidized bed density and a plurality of measuring siten temperatures;
monitoring the recycle stream into the reaction zone where the stream has a reactor inlet temperature;
determining xcex94xcfx81 and comparing xcex94xcfx81 to at least one limit; and
when xcex94xcfx81xe2x89xa710 kg/m3, determining a plurality of An and comparing each An to a lower value and an upper value.
Each of An and xcex94xcfx81 are as defined herein.